With known ship gear train arrangements the differential gearing normally consists of one or more planetary gear trains. Therefore one planetary gear train is provided for forward motion and another planetary gear train is provided for reverse motion.
The main drive motor of the ship can only run and give power when it runs with a certain minimum speed, otherwise it is "strangled." The lowest possible speed of the ship's propellers is given by the lowest gearing of the ship gear train and the lowest motor speed. In order to be able to positively drive the ship propeller smoothly with a still lower speed, down to zero, a hydrodynamic coupling with variable liquid level is arranged in a known position in the main input shaft between ship motor and geared reduction gear. The slippage between its primary part and its secondary part can be varied by changing the filling level (liquid volume) of the hydrodynamic coupling. The minimum slippage occurs at the highest level, i.e. with completely filled coupling. The maximum slippage, in which the secondary part of the coupling no longer rotates at all, occurs when the coupling is completely emptied, in which case the ship propeller is no longer driven by running the main drive motor. One great drawback is that the hydrodynamic coupling must transmit the entire drive torque of the ship and for that reason must be constructed three-dimensionally approximately as large as the mechanical reduction gear train. Another drawback is that the hydrodynamic coupling then also has slippage which means a loss of power between its primary part and its secondary part, even when it is completely filled with liquid.